Electrical insulator having a special external surface configuration for improved performance in contaminated atmospheres

ABSTRACT

An insulator for electrical equipment of the type having a plurality of sheds extending from a central member is provided with a projecting element, or a groove, located between adjacent sheds for defining points between which occurs a controlled uniform discharge whose inception voltage is low as compared with the inception voltage of the discharge otherwise occurring between opposing shed surfaces or between a shed and the central member.

O 1 .7 United States atent 1191 1111 3,829,629 Nitta 1451 Aug. 13, 1974 1 ELECTRICAL INSULATOR HAVING A 3,281,525 10/1966 Sonnenberg et a1. 174/211 SPECIAL EXTERNAL SURFACE FOREIGN PATENTS OR APPLICATIONS CONFIGURATION FOR IMPROVED 260,323 2/1968 Austria 174/211 PERFORMANCE IN CONTAMINATED 1,125,372 7/1956 France 174/209 ATMOSPHERES 1,462,574 11/1966 France 174/211 973,561 3/1960 Germany 174/212 [75] Inventor' Tohe Hyogo Japan 747,131 3/1956 Great Britain 174/212 [73] Assignees Westinghouse Electric Corporation,

Pittsburgh, Pa. Primary Examiner-Laramie E. Askin Filed y 30 1973 Attorney, Agent, or Firm-G. H. Telfer [21] Appl. No: 383,626 [57] ABSTRACT An insulator for electrical equipment of the type hav- 52 US. Cl. 174/212, 174/211 ing a plurality of Sheds extending from a Central 51 11m. 01. HOlb 17/00, HOlb 17/56 her is Provided with a Projecting element, or a groove, 5 Field f Search 174 137 R, 209 210 21 1 located between adjacent sheds for defining points be- 174 212 tween which occurs a controlled uniform discharge whose inception voltage is low as compared with the 5 References Cited inception voltage of the discharge otherwise occurring UNITED STATES PATENTS between opposing shed surfaces or between a shed and the central member. 2,155,848 4/1939 Taylor 174/211 X 0 973,4421 2 1961 Yost 174/209 ux 4 Clalms, 4 Drawmg F gures 1 1 ao I A k A PAIENTED PATENTEB AUG 1 3 I974 smaorz ELECTRICAL INSULATOR HAVING A SPECIAL EXTERNAL SURFACE CONFIGURATION FOR IMPROVED PERFORMANCE IN CONTAMINATED ATMOSPHERES BACKGROUND OF THE INVENTION This invention relates to electrical insulators of the type having a plurality of sheds extending from a central member.

Electrical insulators have been long made in which the surface path from end to end is increased by fins or sheds extending therefrom at axially displaced locations either as separate parallel elements or as a helix. Such insulators are used, for example, between a transmission line and an electrical ground as well as for enclosing components such as lightning arresters. Such apparatus has to remain operable in various atmospheres that may contain moisture and contaminants or pollutants that deposit on the insulator surface and tend to detract from the insulating quality thereof.

In lightning arresters or other equipment shielded by such an insulator, an undesirable internal sparkover of the arrester or insulation failure inside the insulator may occur, particularly in contaminated atmospheres, because of the difference between the potential distribution of the internal elements and the potential distribution of the surface on the outside. What is believed to happen is that as the exterior surface becomes contaminated and wet, leakage current flows over the contaminated surface to a greater degree. The flow of current causes some resistive heating that tends to dry the contaminated surface. This drying process is nonuniform along the surface. For example, it will occur to a greater extent at the bottom of the shed and on the surface of the central member at the base of the shed than in other locations because of greater leakage current or higher current density at the base of the sheds. This non-uniform drying causes a very non-uniform resistance distribution on the surface of the insulator.

The non-uniform distribution of resistance can lead to ultimate failure of the device. When there is a high voltage on the insulator, dry bands on the surface will be shorted by surface discharges that have rather low resistance values. Once the dry bands have been shorted by surface discharges along the entire length of the insulator, the distribution of resistance will be nearly uniform. The device will not fail as long as the surface discharges are divided and kept small by the sheds and leakage current is limited to that from the contamination resistance of the wet top shed surfaces. Failure tends to occur in the course of surface discharge spreading when the dry bands become so large that the surface discharges occur at or near the peak of a sinusoidal voltage applied to power system with which the insulator is associated. Since the surface discharge does not occur until the applied voltage reaches a magnitude close to its peak. the potential distribution of wetted parts, at the top of sheds, separated by dry bands will be determined mainly by stray capacitance of the wet part of the sheds to ground and the line. The potential distribution so determined is not uniform and discharges start only at highly stressed parts. Thus the discharges cause high stress in the vicinity of the discharge.

Discharges at one part of the insulator surface indicate low surface resistances and very high resistance at the parts without discharges. When these processes occur, at or near the peak of the applied voltage, the internal arrester elements are highly stressed. The nonuniform distribution of surface resistances can therefore be the cause of arrester failure by the described process.

Insulators have been designed for use in contaminated atmospheres that are characterized by a greater complexity of the shed configuration such as is disclosed in Standard Handbook for Electrical Engineers, D. G. Fink, Editor-in-Chief, 10th Edition, McGraw Hill Book Company, New York, 1968, pages 13-48 to 13-51, which shows various aspects of prior insulators. The approach taken heretofore in connection with insulators for use in contaminated atmospheres has been to increase the surface leakage or creepage path from end to end by a more complex arrangement of sheds. It has been found that merely increasing the leakage or creepage path does not avoid the problems of nonuniformity of resistance as described above.

SUMMARY OF THE INVENTION The principal purpose of this invention was to design an insulator configuration with improved capability for operating in contaminated atmospheres without failure. The invention is particularly suitable for lightning arrester applications, but may be used in insulators for other purposes such as capacitors and breakers.

In accordance with this invention, an insulator for electrical equipment of the type having a plurality of sheds extending from a central member is provided with means, such as a projecting element or a groove,

located between adjacent sheds for defining points between which occurs a controlled uniform discharge whose inception voltage is low as compared with the inception voltage of the discharge otherwise occurring between opposing shed surfaces or between a shed and the central member.

In the practice of this invention, the projection or groove for defining points between which the discharge voltage is to be made low may be of any convenient dimensional size, substantially small compared with the shed dimensions, as their purpose is to define closely spaced points for controlled breakdown as opposed to being introduced for the purpose of extending the creepage path from end to end of the insulator surface.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a cross sectional view of an embodiment of the present invention; and

FIGS. 2, 3 and 4 are cross sectional views of alternate embodiments of the present invention.

PREFERRED EMBODIMENTS Referring to FIG. 1 there is shown a hollow insulator l0 suitable for enclosing lightning arrester components 20 which may be of conventional configuration and will not be detailed herein. The insulator 10 comprises a central member with a plurality of fins or sheds 14 extending from it at axially spaced locations. The sheds l4 and central member 12 are all integrally unitedas by being formed of a unitary glass or porcelain member. In this preferred embodiment, the sheds 14 extend from the central member 12 substantially parallel to each other although differing in radial extent. Adjacent gnes of the sheds therefore have opposing major suraces 6.

Located between the adjacent sheds is a means 30 for defining points 32 (or surfaces having points 32 thereon) for a discharge whose inception voltage is low as compared with the inception voltage of the discharge occurring between the opposing surfaces 16 of the adjacent sheds 14 or between one of those surfaces 16 directly to the central member 12.

In the embodiment of FIG. 1, the means 30 for defining points 32 for low discharge inception voltage comprises a groove extending within the central member. This groove 30 has a maximum gap between its opposing surfaces that is small, such as less than one-half of the spacing between the opposing shed surfaces 16.

A groove 30 as provided between each pair of adjacent sheds 14 in the preferred-form shown is also provided near the extremities of the insulator beyond the top and bottom sheds 14. The grooves 30 together insure that the starting voltage of external discharges is sufficiently low that the discharges equalize the stress along the surface of the insulator 10 to get nearly uniform potential distribution at the peak of the applied voltage. As discussed in the introduction, this is important so that the low voltage discharges continue until every part of the surface of the insulator is dry and the potential distribution is not drastically effected by contamination resistance. The normally smooth concave surface as occurs between sheds of prior insulators is therefore interrupted to provide the desired small gaps.

FIG. 2 has an alternate form of the invention. Here the means for defining the points for low discharge inception voltage comprises a projection 34 extending proximate the center member from the underside of a shed 14 so that there is a vacant space 30 that defines the points (or surface of points) 32 on the projection and the central member at which the low voltage discharge will occur. It will be understood that the projection 34 of FIG. 2 is preferably provided continuously around the insulator between adjacent sheds and is located between each pair of adjacent sheds. The gap 30A between projection 34 and the adjacent upper shed 14 may also be shaped to assist in achieving low discharge voltage between points 32A of projection 34 and surface 16. v

The-exact location of the groove or projections for defining the low discharge points 32 is a matter of convenience in fabricating the insulator. That is, equivalent functions would be provided by the grooves of FIG. 1 if located proximate a corner between the sheds and the center member. (See FIGS. 3 and 4). The em bodiment of FIG. 1 is preferred for structural stability.

Insulators in accordance with FIGS. 1 and 2 have been fabricated and have been found to significantly improve performance of the insulator under contaminated conditions as compared with prior insulators without the improvement of this invention.

I claim as my invention:

1. An insulator for electrical equipment comprising: a central member with a plurality of sheds extending therefrom at axially spaced locations; axially adjacent ones of said sheds having opposing major surfaces; said central member having, at positions between adjacent sheds, a groove extending therein a distance that is small compared with the radial extent of said sheds and whose maximum axial dimension is small compared with the axial distance between said opposing major surfaces, said groove defining points for low inception discharge voltage as compared with the inception discharge voltage occurring between said sheds in the absence of said groove.

2. The subject matter of claim 1 wherein: said groove has a maximum axial gap less than half the spacing between said opposing shed surfaces.

3. The subject matter of claim 2 wherein: said groove extends within said central member a distance that is less than half the radial extent of said sheds.

4. The subject matter of claim 1 wherein: said central member is hollow and contains lightning arrester components therein. 

1. An insulator for electrical equipment comprising: a central member with a plurality of sheds extending therefrom at axially spaced locations; axially adjacent ones of said sheds having opposing major surfaces; said central member having, at positions between adjacent sheds, a groove extending therein a distance that is small compared with the radial extent of said sheds and whose maximum axial dimension is small compared with the axial distance between said opposing major surfaces, said groove defining points for low inception discharge voltage as compared with the inception discharge voltage occurring between said sheds in the absence of said groove.
 2. The subject matter of claim 1 wherein: said groove has a maximum axial gap less than half the spacing between said opposing shed surfaces.
 3. The subject matter of claim 2 wherein: said groove extends within said central member a distance that is less than half the radial extent of said sheds.
 4. The subject matter of claim 1 wherein: said central member is hollow and contains lightning arrester components therein. 